Premixing device for fluid fuel burners



Dec. 14, 1937. J. w. HAYS 2,102,152

' PHEMIXING DEVICE FOR FLUID FUEL BURNERS Filed Jan. 25, 1933 4Sheets-Sheet 1 Dec. 14, 1937. J. w. HAYS 2,102,152

PREMIXING DEVICE FOR FLUID FUEL BURNERS Filed Jan. 25, 1933 4Sheets-Sheet 2 v DH B p :5 I Q INVENTOII I Dec. 14, 1937. w, A 2,102,152

PREMIXING DEVICE FOR FLUID FUEL BURNERS Filed Jan. 25,1933 4SheefLs-Sheet 3 J K G a \-j I12 WIT 3 ES INVENTOR 7 Dec. 14, 1937. J. w.HAYS PREMIXING DEVICE FOR FLUID FUEL BURNERS 4 Sheets-Shet 4 Filed Jan25, 1933 wn-N s s Patented Dec. 14, 1937 V STATES ears PREMEXING DEVICEFOR FLUID FUEL BURNER/S This invention relates to devices for mixingfuels, as for example liquid, gaseous or finely divided solid fuels withair, and is particularly suitable for use in heating systems of theflameless or surface combustion type.

Surface combustion heating systems operate on the principle that whenfuelmixed with oxy gen is directed onto an incandescent surface,

combustion of the fuel takes place without flame on the surface.

While the principle of surface combustion has long been known, it hasnever been of any commercial importance, because of the fact thatpreviously known systems have been unsafe and uncertain in operation.The greatest danger in the known systems has resided in flash backs andback'firing in the fuel delivering means. In these systems, the practicehas been to mix the fuel and the oxygen at a place spaced from thecombustion chamber and, pass the mixture through a restricted tube athigh'velocity' and pressure into the combustion zone. The tube throughwhich the mixture is injected sometimes becomes highly heated andignites the fuel miX- ture causing explosions at the mixing stage or thesource of fuel supply. Flashback screens and extremely high pressures onthe fuels have been used in order to stop the explosions. Neither ofthese expedients has proven efficient in rendering the systems safe andpractical.

An object of the present invention is the production of a constructionwhich successfully eliminates the danger of explosions in heaters of alltypes wherein mixtures of fuel and air or other combustion supportinggases are fed to the combustion zone.

A further object of the invention is the production of a fuel andcombustion supporting gas injecting device which may be readily used ateither high or low fuel injecting pressures, with resulting economy ofoperation.

The objects of the invention have been achieved through a constructionwhich allows the fuel and air and combustion supporting gas to be mixedonly at or closely adjacent the point or zone of combustion. Morespecifically, this construction may suitable consist of one or more fuelinjecting nozzles through which the fuel alone is injected into thecombustion zone; Supplementing the fuel injecting means are inlets forcombustion-supporting gas through which the gas is led and mixed withthe fuel. The fuel nozzles and combustion supporting gas inlets may beof any suitable proportions for conducting the ele' ments of thecombustible mixture into the combustion zone; preferably they arerestricted in Size and are of a comparatively large number. Such anarrangement makes for good distribution of the fuel throughout thecombustion zone, without the use of excessively high pressures on thefuel if a forced system of feed is used, or excessive draft, if aninduced draft system of feed is used. Either a forced or induced draftmay be used with my mixing device, but preferably afforced system isused with a surface combustion type of heater, because a more equallydistributed combustion zone is formed. Induced draft has a tendency tocause the fuel mixture and resulting heat zones to follow the lines ofleast resistance, thereby causing highly heated channels or lanes ratherthan a uniformly heated zone.

With my mixing device, under no circum stances can explosions take placein the fuel inlets or conduits, because no combustion supporting gas ispresent in them, nor can explosions take place in thecombustion-supporting gas inlet systems, because no fuel is presenttherein.

My fuel injecting and mixing device when used with a surface combustionheater is arranged externally of the combustion zone with the inletnozzles in such a position that the fuel and the combustion supportinggas will be directed against the surface of the refractory mass in thecombustion zone. The fuel and the gas will impinge upon the surface ofthe refractory mass and mushroom, spread and mingle, thereby producing acombustible mixture.

Any suitable means may be used for igniting the gas and fuel mixture,but the igniting means is preferably located at the point where the gasand the fuel mix, that is, immediately adjacent the surface of therefractory mass upon which the fuel impinges.

For a better understanding of my invention,

, reference may be had to the accompanying drawings, in which;

Figure 1 is a cross-sectional view of the mixing device without itshousing.

Figure 2 is a plan view on the line, 2-2, Figure 1.

' Figure 3 is a plan view on the line, 3-3, Figure 1.

Figure 4 is a detail of the tube, N, Figure 1.

Figure 5 is the complete assembly of combustion tube with itsaccessories and the mixing device, all shown in cross-section.

Figure 6 is a plan view on the line, 6't, Figure 5.

Figure 7 is a plan view of the mixer and its accessories, together withthe combustion tube and its accessories.

Figure 8 is an assembly elevation, without detail, showing mixer,combustion tube, etc., together with a. pressure blower.

In Figure 1, P is a gas-supply pipe through which gas is preferablyfurnished under a pressure in excess of the operating pressure and inexcess of the air pressure. Should the static pressure of the gas beless than that of the air it is plainly to be seen that no gas couldflow to the mixer. On the other hand, the air, owing to its much greatervolume, (the proportions being about 10 of air to l of gas when naturalgas is used) may have a pressure in ounces while the gas pressure may bein pounds. High gas pressures, when available, are to be desired becausethey contribute to the capacity of the combustion tube. C is adistribution chamber for the gas before it enters the nozzle tubes N. Bis a bafile to insure distribution of the gas at common pressure to allof the tubes N, but it is not a basic part of my invention. The tubes N,are set gastight in the plate TP, and the latter is secured by the boltsL, to the flange of the detachable housing DH, of the chamber C. G is agasket of any suitable material to make the chamber C gastight. V is aVenturi orifice plate with orifices O, of the well-known Venturi type.One of the offices of this plate is that of a retainer to hold therefractories, Re, in place when the apparatus is operated in ahorizontal position. When the apparatus is operated in a verticalposition with the draft and the mixer at the top, the plate may bedispensed with. While some mixture of the air and the gases is producedin the Venturi orifices the mixture is not completed until the streamsof air and gas are mushroomed by impingement upon the exposed surfacesof the refractory materials. The nozzles of the tubes N, are aligned, asshown in the drawings, with the orifices O. The adjustable spacers S,are used to fix the nozzles N1 of the tubes N at the desired distancesfrom the orifices 0. These spacers may be of any form to satisfy therequirements. In the drawings I show them as rods turned at each end toprovide shoulders, one of the turned-down ends being threaded to screwinto the plate V, and the other passing through TP to which it issecured by the nut Nt, as shown. W is a gasket of the washer type toprevent leakage of gas from C. ST is an equalizer, or straightener. Itsuse will be explained later in this specification. Should it be desiredto change the distances of the nozzles NZ from the orifices O, the nutsNi, may be removed and the spacer-washers SW, or some of them, shiftedto the opposite side of TP, thereby bringing Nl into a closer or moredistant position position with respect to O, as desired.

The above letters and numerals are used throughout the several drawings.

The complete assembly of mixer and combustion tube is shown in thedrawings, Figure 5, in which CT is the combustion tube and Re therefractory material with which the tube is packed. Gr is a casting, inthe form of a grid, which serves to retain the refractory material inthe tube while providing sufficient space for the escape of the wastegases. Gr may be secured in place by any practicable means. J is ajacketing tube surrounding CT and spaced apart therefrom at any desireddistance. Flat metal rings R are welded to J and CT so that the space Spbetween the two tubes is completely enclosed, except for the inlet pipeIn and the outlet pipe 015. The fluid to be heated, Fol, whether water,oil, vapor or whatnot, is caused to flow through Sp, preferably, asshown, in a direction contrary to the flow of the hot gases HG, throughCT.

In the preferred form of construction, which is shown in the drawings,the tube J extends a short distance beyond one end of CT and forms ahousing H, for the mixer, illustrated in Figures 1, 2, 3 and 4. Fl is aflange which is welded to J and at the extreme end thereof. Fl isprovided with bolt-holes, corresponding to those in F and TP, and G1 isa gasket to render the mixer M air-tight. Air under the desiredpressure, whether in ounces or in pounds, is admitted to M through theinlet A. ID is an igniting device, which may be a spark plug, as shown,or any other suitable contrivance.

My invention will be best understood if I next explain the method of itsoperation.

Air in sufiicient volume and at the desired pressure may be supplied bya suitable blower, a compressor or in any other manner. I prefer apressure-volume blower, PB, which is indicated in Figure 8. Whatevermeans may be employed to provide a flow of air, it is highly desirablethat the air be delivered in constant volume and at constant pressure.

The ordinary atmospheric gas burner receives its air supply through anopening cmmunicating direct with the outside air. The flow of air isproduced by lowering the pressure in the furnace below that of theatmosphere, or by the inspirating effect of the gas flowing at a plusatmospheric pressure through an orifice or by both means combined.Burners of this type are sometimes referred to as open system burnersbecause they are subjected to atmospheric pressure at the burner airinlet and to atmospheric pressure again at the stack outlet. My burneris subjected to plus atmospheric pressure at its inlet, applied to boththe air and the gas and to plus atmospheric pressure again in thecombustion zone which is packed with refractory materials amongst whichthe fuel is burned and a great increase in the gas volume takes place.My burner may, therefore, be said to be of the closed system type.

Gas is delivered through the pipe P at a pressure which should beconstant and preferably in excess of that at which the air is supplied.The higher the gas pressure the better for the following reasons:

1. In order to secure the best possible distribution of the gas and thebest possible air-gas mixture, I prefer to deliver the gas through amultiplicity of nozzles Nl. I have found that the best results aresecured when the orifices or vents of these nozzles do not exceed g gndor %l,ths of an inch in diameter. Such small openings under low pressureoperation might become clogged with dust or other foreign materialcarried by the gases, but if the latter are under a considerablepressure head this will be less likely to happen.

2. The higher the pressure of the gases the greater the velocity of theair-gas mixture flowing through the orifices O, and, likewise, thegreater the mushrooming effect when the mixture impinges upon therefractories Re. Such mushrooming, or spreading, promotes a thoroughnessof air-gas mixture. The exposed refractories, i. e., those upon whichthe air and the gas impinge before ignition and upon and among which themixing is completed, are main- I tained in a state of relative coolnessby the air and gas currents.

3. Gas for industrial uses can usually be secured at high pressureswithout increase of cost. Pressure gas, therefore, represents anavailable amount of kinetic energy, costing nothing, and this should beutilized whenever possible. High pressure gas reduces the load on theair blower and thereby lowers the cost of operation;

I do not hold-myself as limited in any way to the number and spacing oftubes N, or orifices 0, shown in my drawings. The number required willdepend upon the size of the combustion tube, the available gas pressureand the dimensions of the nozzle vents. confined to any other particulardetail of any part of the mixing device shown and described herein. Iprefer, as shown in Figure 4, to give the nozzle vents a Venturi effect,so far as practicable.

Air'is delivered to the chamber M through the inlet A. The aircirculates in the space between the housing H and the nozzle tubesN, andfrom this space enters the N tube-bank from all sides. To avoidturbulence at, and an unequal distribution of air to, thenumerousnozzles, the straightener St may be used to straighten the air flow andbreak it up into a series of currents running parallel with the tubes N,which are severally aligned with the'orifices 0. S t serves to equalizethe distribution of the air and air pressure and preventshort-circuiting to the nearest orifices. St is merely an orifice platea with holes in alignment with the orifices 0. It is adjustable as toposition by means of the spacers Si, which are washers of variousthick-.

nesses, and the space desired may be arrived at by increasing. ordecreasing the number of washers. I find, in practice, that verysatisfactory results are secured by positioning the straightenerapproximately as shown in my drawings. The straightener may beconsidered as a desirable accessory, but by no means a basic part of myinvention.

The air and gas are partially mixed in the orifices O. The gas, beingunder relatively high pressure, is projected at high velocity and in adiverging cone into each orifice, drawing air with it from thesurrounding stream of air. The air, being under pressure, flows underits own velocity head into the gas stream. The velocity in the orificeis, therefore, a composite of the velocities of the air and the gas, andthis velocity is accentuated by the Venturi effect of the orifice.Mixture is completed to a point of extreme thoroughness because of themultiplicity of the gasair stream, and because of the mushrooming effectwhen these streams impinge upon the broken surfaces of the refractorymaterials R. The gases are ignited in the vicinity of this point ofimpingement by means of an ordinary sparkplug, or other igniting deviceID. It will be seen that a flash-back is impossible for the reason thatthe point of ignition lies in, or near to, the very locality where thegas and the air are brought together to form a combustible mixture.There is noreason for a screen or other antifiashback device, becausethere can be no flashback into an atmosphere consisting entirely of airor entirely of gas.

Immediately after ignition the mixture burns in the interstices of therefractory packing. at and beyond thepoint of ignition and in a shorttime the refractories become heated to a point of incandescence,whereupon the mixture burns Neither do I hold myself as without flameand a very high temperature is reached, the heat being produced, verylargely if not entirely, in the radiant form. Combustion ispracticallyinstantaneous. This being so, the capacity limits of such a combustiontube are fixed by the free area of the interstitial spaces of therefractorypacking and by the pressures used to force the gases into andthrough the tube.

The combustion'zone, or hot-spot, is indicated in Figure by the dottedlines, CZ. The transfer of heat from the incandescent refractories Re inCZ, to and through the tube CT, to the fluid Fl, is almostinstantaneous, the heat being in the radiant form. CT may be packed withthe refractories Re throughout its entire length, if consideredadvisable. The hot gases leaving CZ are baffled by these refractories,and brought into contact repeatedly with CT, giving up their heat asthey flow. Furthermore, these gases heat the refractories themselves andthe heat is transferred by radiation and conduction from one refractoryelement of the packing to another and finally to the tube CT, throughwhich it is conveyed to the fluid in the space Sp. In this way a veryrapid and very complete transfer of heat is effected.

The positioning of the combustion zone CZ is a very importantconsideration in all cases where such combustion tubes are employed. Theadvantages of counter-flow in effecting heat exchanges are well known.The fluid to be heated should, in all cases where possible, flow in adirection contrary to that of the hot gases. This is best accomplished,as in the present instance,

by fixing the location of the combustion zone at the extreme end of thecombustion tube CT, and approximately adjacent to the place Where theheated fluid reaches its outlet. In this ,way the fiuid, when in itshottest state, is exposed to the highest temperature and when in itscoolest state, on entering the heater, is exposed to the lowesttemperature. Thus the highest heater capacities and efficiencies areobtained.

It can easily be seenthat if the combustion zone should be establishedat the opposite end of the combustion tube, or midway of the tube, therewould be a marked loss of heat energy with the waste combustion gases,and this would be refiected adversely upon the heating device, both asto its efiiciency and its capacity.

Previous inventors, as already explained, have ignited the air-gasmixture at the discharge outlet of the combustion tube, relying upon abalancing of the velocities of mixture flow and of flame propagation tofix the location of the combustion zone, but I have found that thecombustion zone is likely to remain fixed, at, or close to, the spotwhere it is first established. The tendency of the hot-spot to remainstationary is influenced to some extent by the state of therefractorypacking, as to coarseness or fineness, by the length of the tube and bythe temperatures attained in the hot-spot as well as by the velocities.There is an enormous increase in gas volume in the combustion zone, dueto the high temperature, and this acts as a brake upon the flow of thegases through the tube.

My invention enables me, among other things. to fix the combustion zoneat the forward end of the combustion tube, or at any other point where Imay prefer to ignite the mixture, and to maintain said zonesubstantially in its original location and position.

I accomplish this by igniting the mixture at the forward end of the tubeby a spark plug, as shown in my drawings, or otherwise.

While I have shown tubes in my drawings and referred to them as such inthis specification, I do not hold myself as limited to tubes. Any formof chamber or receptacle may be used to hold the refractory materials orthe fluid which is to be heated, whether that form be tubular orotherwise, as it is plain that I can vary the size, shape and materialsof any and all of the structures shown and claimed without departingfrom the spirit of my invention.

I have found that under some circumstances, as already stated theorifice plate V may be omitted, but where the air pressures are low itis of a distinct advantage.

I have found that there is a distinct advantage in using a number ofpipes N, as this gives a better distribution of the combustible mixtureacross the face of the refractories Re, in the tube CT. The higher thegas pressures the smaller the number of tubes N which are required.

I claim:

1. In a device for delivering a combustible mixture of a fuel andcombustion-supporting gas to a furnace chamber containing an aperturedrefractory mass, the combination of a plurality of fuel injectingnozzles directed into the furnace adjacent said refractory mass, meansfor supplying fuel under pressure to said nozzles, means for directing acombustion-supporting gas into the furnace chamber adjacent the nozzles,whereby the fuel and the combustion-supporting gas impinge upon therefractory mass and are thoroughly mixed and means for igniting themixture initially at the point of mixture.

2. In a device for delivering a combustible mixture of fuel andcombustion-supporting gas to a furnace chamber containing an aperturedrefractory mass, the combination of a plurality of fuel injectingnozzles directed into the furnace chamber adjacent said refractory mass,means for supplying fuel under pressure to said nozzles to impinge thefuel on the surface of said refractory mass, means for directing acombustion-supporting gas around said nozzles and impinging it on therefractory mass, whereby the fuel and said gas will be mixed, and meansadjacent the refractory mass for igniting the mixture of fuel and gas.

3. In a device for delivering a combustible mixture of fuel andcombustion supporting gas to a furnace chamber containing an aperturedrefractory mass, the combination of a fuel distributing chamber, meansfor supplying fuel to said chamber under pressure, a plurality of fuelinjecting pipes connected to said distributing chamber, nozzles on saidpipes directed into said furnace chamber adjacent said refractory massfor impinging the fuel against said refractory mass, means for directinga combustion-supporting gas around said nozzles and impinging it on saidrefractory mass, whereby said fuel and said gas will be thoroughly mixedand means for igniting the mixture at the point of mixture.

4. In a device for delivering a combustible mixture of a fuel and acombustion-supporting gas to a furnace chamber containing an aperturedrefractory mass, the combination of a plurality of fuel injectingnozzles directed into said furnace adjacent said refractory mass, aplate between said nozzles and said refractory mass, apertures in saidplate in alignment with said nozzles, means for delivering a combustionsupporting gas around said nozzles and through said apertures, means forsupplying fuel under pressure to said nozzles, whereby the fuel will bedirected through said apertures, at least partially mixed with said gas,impinged upon said refractory mass and mixed, and means for igniting themixture substantially at the refractory mass.

5. A flameless combustion heating device comprising a furnace chamber,an apertured refractory mass therein providing a surface for flamelesscombustion of fuel, means for directing a plurality of streams of fuelagainst said refractory mass, from a point exterior of the furnacechamber, means for directing a combustion-supporting gas against saidrefractory mass, whereby said fuel and gas are thoroughly mixed, andmeans for initially igniting the mixture at the point of mixture.

JOSEPH W. HAYS.

